Minimal Aerosol Loading and Global Increases in Atmospheric Ozone during the 1996–1997 Martian Northern Spring Season

Abstract

We employed the Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) to observe middle ultraviolet (220–330 nm) spectral scans of Mars for two periods during the 1996–1997 northern spring season. Analysis of these data yields quantitative measurements of atmospheric column opacities for aerosols and ozone over a wide range of latitudes (40°S–70°N) in two Mars seasons (solar longitudes,Lsof 10° and 61°). The most significant findings of the analysis are: (1) the global Mars atmosphere exhibited minimal aerosol loading during this season (variable opacities of 0.02–0.08 for clouds, or 0.05–0.2 for dust); (2) the low-to-mid latitude ozone abundance in the Mars atmosphere increased by roughly a factor of two (from 1.8 to 3.6 μm-atm) betweenLs=10° andLs=61°. These results support previous predictions (R. T. Clancy and H. Nair 1996,J. Geophys. Res.101, 12785–12790) and observations (R. T. Clancyet al.1996b,J. Geophys. Res.101, 12777–12783) of orbital (Ls) variations in Mars global ozone abundances, as driven by orbital variations in the global altitudes of water vapor saturation (R. T. Clancyet al.1996a,Icarus122, 36–62). They also support conclusions of very low dust levels in the Mars atmosphere around aphelion (Clancyet al.1996a), with upper limits for background dust opacities which are several times lower than described for this season by the Viking-based dusty model of the Mars climate (e.g.,τ<0.2, here; versus 0.6; D. S. Colburnet al.1989,Icarus79, 159–189). Also of interest is the observation of substantially reduced cloud opacities for the “aphelion cloud belt” in early northern spring (Ls∼ 60°) in 1997, versus 1995 or 1991.